This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-296964, filed Sep. 28, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an antenna apparatus of a portable radio terminal having a radio function.
2. Description of the Related Art
In a portable radio terminal including an antenna and a radio that are integrated as one component, the directivity of the antenna is likely to vary with the shape of a cavity.
It is known that the use of a half-wave (xcex/2) antenna reduces current flowing from a feeder line to a cavity. If a half-wave linear antenna is employed, an influence of a cavity is reduced, which is convenient for the design of an antenna system.
Even in the half-wave antenna, however, current is likely to flow into the cavity to make it impossible to form a desired pattern depending on the configuration of a matching circuit.
In most cases, a quarter-wave (xcex/4) element is used as the matching circuit for impedance matching between a half-wave antenna and a load. It is known that the quarter-wave-length element has relatively wider band characteristics than the matching circuit of lumped constant elements does. However, there is a problem that current leaks from a feeder point to a cavity when the quarter-wave-length element is inserted between the half-wave antenna and the feeder point. Such a leakage current causes unnecessary electric wave emitted from the cavity.
Conventionally, when the quarter-wave-length element was used as a matching circuit, its wide-band characteristics were prioritized and a leakage of current to the cavity had to be ignored. In other words, an attempt to optimize the quarter-wave-length element was made to obtain wide-band characteristics, not to reduce a leakage of current to the cavity.
In Japanese Patent Application No. 11-051462, the inventors of the present invention proposes an antenna apparatus in which a connecting point between a half-wave antenna and a quarter-wave-length element serving as a matching circuit is located in a specific space of a cavity to reduce a leakage of current to the cavity.
FIG. 8 is a schematic diagram of the antenna apparatus proposed in the above Japanese Patent Application. Referring to FIG. 6, a connecting point 65 is located close to the side of a cavity 61 and somewhat below a feeding point 62 within a range of the side. This configuration produces the advantage of reducing a leakage of current to the cavity.
However, the antenna apparatus shown in FIG. 8, of which the quarter-wave-length element serving as a matching circuit has to be encased in a plastic cavity of a cellular phone, the quarter-wave-length element becomes relatively long depending on the frequency and thus requires a large area when it is inserted in the cavity of a cellular phone that steadily decreases in size. Further, the wide-band characteristics of the quarter-wave-length element are likely to deteriorate.
An object of the present invention is to provide an antenna apparatus capable of achieving compatibility between a reduction in mounting area and an increase in frequency band of a matching circuit thereof.
Only the physical length of a quarter-wave (xcex/4) element serving as a matching circuit is decreased without changing the electric length thereof. The electric length of the quarter-wave-length element corresponds to a length of xcfx80/2 in which the amplitude of an electromagnetic wave reaches its peak, while the physical length corresponds to the actual length of the quarter-wave-length element measured from the feeder point.
Specifically, an antenna apparatus of the embodiment of the present invention is coupled to a feeding point arranged on a cavity. The antenna apparatus includes a linear element extending from the feeding point and having a physical length that is shorter than a one-quarter wavelength, a half-wave antenna element connected to an end of the linear element, and a metal conductor piece including a connecting point between the linear element and the half-wave antenna element and located close to one side of the cavity.
The linear element whose physical length is shorter than a quarter wavelength includes at least two portions as to an imaginary plane in parallel with one side surface of the cavity. The one is a first portion extending in a first direction opposite to that of the half-wave element from the feeding point. The other is a second portion extending in a second direction, which is equal to a direction in which the half-wave antenna element extends.
The linear element and the half-wave antenna element are connected at their ends. This connecting point may be located bellow the feeding point along in the first direction.
The physical length of the linear element is shorter than a one-quarter wavelength, while the electric length thereof is a one-quarter wavelength. Therefore, the end of the linear element needs to be capacitive.
The metal conductor piece attached to the end of the linear element has only to be thicker than the linear element and is shaped like a cylinder, a sphere, a square pole and the like. The whole or only the surface of the metal conductor piece can be formed of a conductor. This metal conductor piece increases the capacitance of the linear element. The capacitance is in proportion to the cross-sectional area of the conductor and in inverse proportion to the length thereof. The cross-sectional area of the metal conductor piece is therefore larger than that of the linear element. The physical length of the linear element can be shortened by the capacitance corresponding to an increase in cross-sectional area. More specifically, the physical length of the linear element can be reduced to a range from xcex/6 to xcex/5.
The metal conductor piece forms a plurality of current paths between the linear element and the half-wave antenna element. The frequency band of the matching circuit of the antenna apparatus can thus be broadened. Consequently, the lateral area of the metal conductor piece per unit length is larger than that of the linear element per unit area.
A dielectric may be inserted between the metal conductor piece and one side of the cavity. In this case, the dielectric serves to increase the capacitance of the metal conductor piece. If the capacitance increases, the physical length of the linear element can be shortened further, and the mechanical strength between the metal conductor piece and the cavity can be increased.